Chemistry - Unit 12: Experimental Techniques and Chemical Analysis

what apparatus is appropriate for measuring time?

stop-watches

what apparatus is appropriate for measuring temperature?

thermometer

what apparatus is appropriate for measuring mass?

balances

what apparatus is appropriate for measuring volume?

• burettes (measuring volume of a liquid to the nearest 0.00cm³)

• volumetric pipette (measuring a specific, fixed volume of a liquid)

• measuring cylinder (measuring approximate volume of a liquid)

• gas syringe (measuring volume of a gas)

state the advantages and disadvantages of stop-watches?

advantages - can measure time intervals to a high level of precision.

disadvantages - accuracy of stopwatch reading is dependent on the reaction time of the user so human error is introduced.

state the advantages and disadvantages of thermometers?

advantages - easy to use and cheap to acquire

disadvantages - limited temperature ranges, lower precision compared to a digital temperature probe and parallax error can occur

state the advantages and disadvantages of measuring balance?

advantages - speed readings to a high level of accuracy

disadvantages - sensitive to environmental factors such as air currents and fluctuations in temperature in the lab

state the advantages and disadvantages of burette?

advantages - wide range of burettes so a range of accurate volumes can be measured

disadvantages - meniscus reading error can occur if user does not view the measurement at eye level

state the advantages and disadvantages of volumetric pipettes?

advantages - can accurately measure a specified volume consistently

disadvantages - limited to measuring only one fixed volume

state the advantages and disadvantages of gas syringes?

advantages - can measure volumes of a gas to a high levwl of accuracy

disadvantages - gases in the closed system can become sensitive to environmental factors such as changes in temperature or pressure

how to comment on the advantages and disadvantages of an experimental method?

ask the following questions:

• what are the advantages of the apparatus used?

• how many repeats have been done? were the answers similar or different? the results are more precise if the repeat readings are closer

• were there any controlled variables to ensure that the independent variable is the only factor causing a change?

what is a solvent?

a substance that dissolves a solute

what is a solute?

a substance that is dissolved in a solvent

what is a solution?

a mixture of one or more solutes dissolved in a solvent

what is a saturated solution?

a solution containing the maximum concentration of a solute dissolved in the solvent at a specified temperature

what is residue?

a substance that remains after evaporation, distillation, filtration or any similar process

describe an acid-base titration

1. use the pipette and pipette filler and place exactly 25 cm³ sodium hydroxide solution into the conical flask

2. place the conical flask on a white tile so the tip of the burette is inside the flask

3. add a few drops of a suitable indicator to the solution in the conical flask

4. perform a rough titration by taking the burette reading and running in the solution in 1 – 3 cm³ portions, while swirling the flask vigorously

5. quickly close the tap when the end-point is reached (sharp colour change) and record the volume, placing your eye level with the meniscus

6. now repeat the titration with a fresh batch of sodium hydroxide

7. as the rough end-point volume is approached, add the solution from the burette one drop at a time until the indicator just changes colour

8. record the volume to the nearest 0.05 cm³

9. repeat until you achieve two concordant results (two results that are within 0.1 cm³ of each other) to increase accuracy

what is the colour of litmus solution in acid, alkaline and neutral?

colour in acid: red

colour in alkaline: blue

colour in neutral: purple

what is the colour of red litmus paper in acid, alkaline and neutral?

colour in acid: stays red

colour in alkaline: turns blue

colour in neutral: no change

what is the colour of blue litmus paper in acid, alkaline and neutral?

colour in acid: turns red

colour in alkaline: stays blue

colour in neutral: no change

what is the colour of methyl orange in acid, alkaline and neutral?

colour in acid: red

colour in alkaline: yellow

colour in neutral: orange

what is the colour of phenolphthalein in acid, alkaline and neutral?

colour in acid: colourless

colour in alkaline: pink

colour in neutral: colourless

what is the colour of thymolphthalein in acid, alkaline and neutral?

colour in acid: colourless

colour in alkaline: blue

colour in neutral: colourless

describe how paper chromatography works

• paper chromatography is used to separate substances that have different solubilities in a given solvent.

• pencil line is drawn on chromatography paper and spots of the sample are placed on it. pencil is used for this as ink would run into the chromatogram along with the samples

• the paper is then lowered into the solvent container, making sure that the pencil line sits above the level of the solvent so the samples don´t wash into the solvent container

• the solvent travels up the paper by capillary action, taking some of the coloured substances with it

• different substances have different solubilities so will travel at different rates, causing the substances to spread apart. those substances with higher solubility will travel further than the others

• this will show the different components of the ink / dye

how can paper chromatography be used to identify unknown substances?

• compare the chromatogram of the unknown substance with chromatograms of known substances.

• if the chromatograms between the 2 substances are identical, they are the same substance

how can paper chromatography be used to identify whether a substance is pure or impure?

• the chromatogram of a pure substance will only have one spot

• the chromatogram of an impure substance will have multiple spots show up

describe the use of locating agents.

colourless mixtures of chemicals can be analysed if the ‘spots’ can be coloured by a chemical or light treatment.

state the equation for R_f

R_f = distance travelled by substance ÷ distance travelled by solvent

describe and explain how a mixture can be separated using a suitable solvent.

• a solvent is a substance that dissolves a solute

• the type of solvent must be carefully chosen depending on the type of solute that needs to be separated to ensure no other impurities or components dissolve in the solvent and only the desired substance is dissolved in the solvent.

describe and explain how a mixture can be separated using filtration.

• filtration separates an insoluble solute from a solution

• filter paper is placed into a funnel, the small pores only allow liquids to pass through

• the mixture is poured into a beaker through the funnel, leaving the solute behind

describe and explain how a mixture can be separated using crystallisation.

• crystallisation separates a soluble solute from the solution it is dissolved in

• heat the solution in an evaporating dish over a bunsen burner until boiling (specify that no more than half of the solution is boiled off before it is removed from the heat)

• remove from the heat and allow the solution to cool and evaporate

• the saturated solution will leave behind crystals as it cools

• the crystals will grow and can be collected and allowed to dry

describe and explain how a mixture can be separated using simple distillation.

• simple distillation separates a solvent from a solution

• simple distillation works because the dissolved solute has a much higher boiling point than the water so the water will boil and evaporate first and the salt stays behind

• the water vapour will cool and condense as it travels through the condenser, collecting in a separate flask

• the remaining solution becomes more concentrated in solute as the amount of solvent in it decreases

describe and explain how a mixture can be separated using fractional distillation.

• fractional distillation separates a pure liquid from a mixture of liquids

• fractional distillation works because the liquids have different boiling points

• the mixture is heated to the boiling point of the liquid with the lowest boiling point, which will evaporate first, pass through the condenser and collect in its fraction. this process is repeated, until every component of the mixture has evaporated in its associated boiling point and separated into its individual fractions.

how to identify the purity of a substance using their melting and boiling point information

pure substances have a sharp, exact melting and boiling point whereas impure substances will melt/boil over a range of temperatures

test and result for carbonate (CO_3^2- )

test

result